1. Field of the Invention
The present invention relates to inorganic phosphors that produce full-spectrum down-converted white-light in the presence of a semiconducting element.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Solid-state lighting based on phosphor-converted light-emitting diodes (pc-LEDs) is bound to replace low-efficiency incandescent as well as mercury-containing fluorescent light sources due to the promise of lower energy consumption and longer lifetimes [1-6]. Among the many classes of materials that have recently been investigated for potential lighting applications, a significant amount of the work on oxide hosts is carried out on aluminates[7-9], aluminosilicates[10,11] and silicates [12,13].
In order to replace inefficient incandescent lighting, much effort has been spent to develop new materials that show higher efficiencies and better (long-term) stability compared to incandescent light bulbs and also compact fluorescent lamps (CFL). Therefore, white light generation based on solid-state devices is considered a promising alternative due to efficiency gains as well as longer lifetimes and eventually lower operating costs. Potentially, there are several different ways to create white light by solid-state means: (i) by employing a red, green and blue light-emitting diode (LED), (ii), using a blue LED that excites a luminescent material (a phosphor) that down-converts part of the blue light to yellow-green radiation or (iii) by utilizing a (near)-UV LED with an emission wavelength of about 350-400 nm that subsequently excites one or more phosphors that then create the desired white light. Due to some inherent and technical limitations, most solid-state lighting devices are based on the latter two mechanisms; because of the involvement of a down-converting material, usually they are referred to as phosphor-converted light-emitting diodes (pc-LEDs).
Evolution of the pc-LED originated with a yellow emitting phosphor cerium-doped yttrium aluminum garnet (Y3-xCexAl5O12) potted on a blue emitting semiconducting element. A combination of “yellow” photoluminescence with “blue” electroluminescence produces a composite white light (yellow-green). The composite white light typically has poor color quality and cool color temperature, exhibiting a spectral maximum around 550 nm. Due to the lack of a red component in the emission spectrum, the white light appears blue-ish to the human eye and is therefore considered “cold” white light.
Improvements to the next generation of the pc-LED focused on improving the color quality by introducing a secondary phosphor to manipulate the color coordinates, improving the color rendering and temperatures. Relying on a mixture of red emitting and green emitting phosphors potted on a blue semiconducting element, the composite white light can now be manipulated by controlling the intensity of each emitting primary color (red, blue and green) exhibiting a spectral maximum ranging from 525 nanometers (nm) to 680 (nm). While gains were made on the quality side, new engineering considerations were revealed on the efficiency side; namely internal reabsorption of green photoluminescence by the red emitting phosphor and scattering of the lower wavelength photons.
Further improvements to color quality and control over color temperature has resulted in a three component phosphor blend; red emitting, blue emitting, and green emitting phosphors potted on a near-ultraviolet (UV) semiconducting element.
A more efficient approach to pc-LED architecture is to have a single component phosphor emitting full-spectrum photoluminescent white light through excitation by a near-UV semiconducting element. Potential advantages of single-phase materials over blending materials together is the potentially better stability (both chemically as well as in terms of color) as well as the absence of reabsorption of parts of the emitted light from the green and red components of the phosphor blend. This can eventually lead to better luminous efficiency as well as better color rendering index, Ra.
Within the silicates, Ba3MgSi2O8 has been the focus of much research effort since the late 1960's [14,15], mostly due to its very good thermal stability, high quantum yield, abundant constituent elements and the potential for full-color emission. Phosphors based upon Ba3MgSi2O8 co-activated with Eu2+ and Mn2+ emitting blue and red light have been studied extensively. However, after four decades of research, no true full-color emitting phosphor based on Ba3MgSi2O8 has been reported. Earlier reports of tri-band (i.e. blue, green and red) luminescence [12,16,17] from Eu2+ and Mn2+ co-activated samples have been demonstrated, resulting from significant amounts of orthosilicate (Ba2SiO4) impurities, which exhibits the observed broad and very efficient emission band centered around 505 nm [18,19].
The elucidation of the structure of Ba3MgSi2O8 has also been in the center of much discussion. Originally described as iso-structural to the calcium analogue Merwinite (Ca3MgSi2O8) by Klasens et al. [20], it has later been found to be closely related to the Glaserite structure type of K3Na(SO4)2 [19,21]. Recently, Park et al. [22] have used a combined neutron and X-ray diffraction study to determine the true unit cell. It was found that Ba3MgSi2O8 crystallizes in a trigonal space group P-3 (space group 147) with the cell parameters being a,b=9.72411 Å and c=7.27647 Å.
As for most oxide phosphors, the preparation usually relies on the treatment of intimate mixtures of solid precursors in high temperature furnaces, often under reducing atmospheres to stabilize the desired valence state of the dopant ions (such as Eu2+ and Ce3+). Microwave-assisted preparations, reported as early as in the 1980s [23,24], offer a very rapid and energy efficient alternative to more classical pathways. Recently, several different materials could be prepared employing microwave-assisted solid-state pathways, such as Skutterudites [25], intermetallics [26] and oxide phosphors [27-30].